Contour indicating device



A. v. BEDFORD CONTOUR INDICATING DEVVICE Nov. 8, 1949 3 Sheets-Sheet lFiled May 21, 1947 ,m M Y M w o mM m Am Am Nov. 8, 1949 A, v, BEDFORD2,487,511

CONTOUR INDICATING DEVICE 5 Sheets-Shea?I 3 Filed May 2l, 1947 .1j-5lATTORN EY to such a device in which tude proportional to a formed on thetarget, the

. instead ofa cathode ray, i paper, or other ordinates. By meansPatented Nov. 8, 1949 am Apostol-d. Princeton. N.

Radio Corporation of America,

of Delaware J., assigner to a corporation application Mu 21. 1941,serial No. '749,569' n calma. (ol. :i5-ss) 41 This invention relates toan indicium, and particularly an electrical current is produced whoseamplitude or frequency varies in accordance with the eontor of apredetermined curve or area.

In its broader aspect the invention has utility in various fields ofapplication. For example, electronic computing systems are known whereina cathode ray is normally deilected to a reference point near one-corner of a target provided with secondaryelectronemissivev curvesarranged in accordance with a particular function of two variables, zand u. The ray is deflected along the :s rectangular coordinate adistance determii'ied by a tional to a selected value of r. The ray isthen deflected along the perpendicular u coordinate a distancedetermined by a voltage of an amplito the contour -of secondary-emissivelines number crossed at any position on the :c coordinate beingdetermined by the configuration of the lines in accordance with theHpredetermined function. As each line is crossed a pulse is producedwhich operates a counter whereby a representation of the value of thefunction f(z,u) is established. (See U. S. Patent vl3,412,467, issuedDecember 10, 1946, to George A. Morton.)

One disadvantage of such a system is that the function curves on thetarget are necessarily enclosed-within the cathode ray tube and thuscannot be f changed. Furthermore, it is sometimes deshable to produce.an output voltage whose amplitude or frequency is proportional to thedesiredfunction, rather than a number of pulses.

The-'present invention utilizes a beam of light, which scans a target ofsimilar material, on which may be drawn or printed a curve representingthe particular function u=f(z) in4 rectangular coof a conventionalvibrating mirror the light beam'is caused to sweep cyclically across thecurve in a direction parallel to the u coordinate. Its position alongthe x coordinate may be manually adjusted to a position correspending toa given value of the variable a', or deflected electrically by a secondmirror, to the desired position on the :r coordinate. i. The presentinvention then provides means forv producing a D. C. voltage whoseamplitude or an alternating voltage whose frequency distance from the a:coordinate to the curve, and

crosses a number of apparatus for the` .production of an egectricalsignal corresponding selected value of y, andvoltage 'of an amplitudeproporlight, parallel to producing a third is proportional to .the

thus is indicative of the value of y in the equation 1u=f(:c). A

It will be observed, therefore, that the present invention providesmeans for producing an electrical signal corresponding, in amplitude orire-.- quency to the contour of a line, or any similar indicium, as aspot of lightloscillating in a direction parallel to the y coordinate ismoved along the :i: coordinate.

This invention also has utility as a reading aid for the blind, and apreferred embodiment of the invention having -particular utility forsuch a purpose will b e described in detail. When applied to thesolution of this problem, the indicia scanned by the light beam will bethe vprinted characters, letters or numerals, of a text, uniqueidentication of their outline being accomplished by vibrating the spot,produced by -a beam of the u axis, that is, transversely across the lineof characters, at a relatively rapid rate while moving the spot moreslowly along the line (the a: coordinate) to produce an audible tone.The frequency of the tone is indicative of the distance from the:lr/axis, which may be considered as an imaginary n reference axis aboveor` below the line of characters, to the edge of the character at thepoint in question. The varyingvfrequency produced as the line is readpermits a visual picture to be formed indicative of the shape or contourof the top or bottom of the individual characters. To provide a morepositive identication, the top and bottomcontours may be reproducedsimultaneously in terms of two separate and distinct tonesvaryingsimultaneously in frequency.

The upper and lower contours of a few characters may be substantiallyidentical. For their positive identification there is provided means fortone of constant frequencythe presence of which indicates that a portionof the character lies in an area near the horizontal center line of thecharacters. This will permit the identification of such letters as 0, e"and c for example.

It is, therefore, the primary object of this invention to provide meansfor producing an electrical signal whose frequency or amplitudecorresponds to the contour of an indicium such as a line, letter,numeral vor distinctive area.

Itqis a further object of this invention toprovide means for scanning agiven area containing information-representing indicia and for producinga signal whose amplitude or, frequency variiaes in accordance withthe'contour of said indic A still further object oi this invention is toprovide an improved reading aid for the blind.

A still further object is to provide means for producing an audible tonerepresentative of the tion both as to its organization and method ofoperation, as well as additional objects and advantages thereof, willbest be understood from the following description when reali` inconnection with the accompanyingf drawings, in which Figure 1 is adiagrammatic drawing, partly in block diagram form, of an embodiment ofthis invention;

Figure 2 is a set of curves illustrating the oper- -ation of the deviceillustrated in Fig. 1;

Figures 3 and 3a illustrate the contours of lower case and capitalletters, respectively, showing the corresponding frequency variationproduced by the device illustrated in Fig. 1;

Figure 4 is an enlarged view of the scanning path followed by the lightsp'ot during two successive cycles across the letter "E;

Figure 5 is the circuit diagram o f a multivibrator which may be used inconnection with the invention;

Figure 6 is the circuit diagram of a D. C. setter;

Figure 7 is the circuit diagram of a clipper;

Figure 8 is the circuit diagram of a keyer; and

Figure 9 is the circuit diagram of a peak detector.

Referring to Fig. 1, the apparatus for scanning the printed iinewith aspot of light is diagrammatically illustrated, since this portion of theapparatus is conventional. The probe or scanner may take the formillustrated in acopending application of Zworykin et al., Serial No.560,472,

iled October 26, 1944, now Patent No. 2,451,014. A source of light I isfocussed by a lens 3 on the printed pape 5 which contains the line ofcharacters to be read," illustrated in greater detail in Fig. 3, by wayof a mirror 1 mounted for rotational movement about a horizontal axis,in response to the energizing current flowing in coil 9. The lightreflected from the page is picked up by a phototube II. In the viewshown in Fig. 1 the vibration of the mirror causes the small spot oflight to sweep transversely across the printed line, which is assumed toextend in a direction perpendicular to the drawings.

The electrical components for the apparatus are shown in block diagramform. Those elements which are conventional and well known in the'arthave not been illustrated in detail, but those which may not be wellknown, or which require specific description of their mode of operationor method of connection will be illustrated in detail in subsequentfigures of the drawing.

4A master multivibrator clipper and amplifier device I3 provides theprimary synchronizing voltage which controls the operation of theelements of a combination. This is simply a well known device of thetype which produces a cyclic square wave output voltage of the typeillustrated in curve a of Fig. 2. The square wave voltage from themaster multivibrator I3 is applied to the inputv of a sweep integratorI5 which converts the square wave voltage into a symmetrical sawtoothvoltage of the type illustrated in curve c of Fig. 2. The output of thesweep integrator I 5 is applied to the energizing coil 9 which causesthe linear re ciprocal vibration of the mirror 1.

The square wave voltage produced by the master multivibrator I3 is alsoapplied to the input of a control wave diiferentiator l1 which producesan output voltage of the type shown in curve b oi' Fig. 2. The output ofthe photoelectric tube Il is applied to the input of a signal amplifierI3, which ampliiies the voltage produced by the phototube and produces asignal voltage which is characteristic of the black surface of theprinted characters along the line traversed by the spot of light duringany cycle of operation.

For the purpose of illustration two complete cycles have been shown inthe curves of Fig. 2 based on the assumption that the probe is scanningthe letter E, as shown in Fig. 4. It is assumed that for the period inquestion, the light spot starts at point 2l, moves transversely acrossthe letter to point 23 and returns to point 25 in the first cycle ofoperation. It will be noted that during Ithis period the probe has beenadvanced along the line a'short distance. It is further assumed thatduring the rst cycle the spot crosses the top and bottom of theletter aswell as the central portion 21. It is further assumed that during thesecond cycle of operation the spot moves from point 25 topoint 29andback to point 3|, but that it has now moved to the right a sumcientdistance so that the spot does not strike the central portion 21 of theletter.

The potential of the voutput voltage produced by signal amplifier I9 isso controlled that a positive output potential corresponds to black andzero potential corresponds to white Thus it may be observed that theirregular square wave voltage of curve d in Fig. 2 represents the outputvoltage of the signal amplifier l! during the two cycles of operationdescribed in connection with Fig. 4. v

An upper envelope multivibrator 33 is a device having two stableconditions of operation and may be, for example, of the type illustratedin Fig. 5. This device includes two tubes 35 and '31 which areinterconnected in such a manner that when one tube is conducting itbiases oi the other tube, and the condition can'only be reversed byapplying a positive pulse to the grid of the other tube. This pulse thencauses the other tube to conduct and it in turn biases 0H the firsttube. Two input terminals 39 and 4| are therefore provided, output beingtaken across the cathode 'resistor of tube 35 and is available at outputterminal 43.

The signal voltage d is applied to input terminal 39 while thedifferentiated voltage produced by control wave differentiator II isapplied to input terminal 4I. Assuming that prior to the arrival of thefirst positive pulse 45 (Fig. 2) of the differentiated wave b that tube35 was conducting, the pulse 45 will cause tube 31 to conduct and tube35 Will be cut oi. Simultaneously the spot will begin to move from point2l to point 23 (Fig. 4). Consequently, the output voltage (curve e) willdrop to ground potential. This condition will continue until a positivepulse is applied to the grid of tube 35, which will occur when the iightspot reaches the first black area of the letter. The resulting pulse 41will reverse the conditionof conductivity and the output voltage atterminal 43 will rise to a relatively positive value due to the platecurrent in the cathode resistor of tube 35.

, tiator 5l, the output The capacitor is again applied to i amplitude ofthe thev capacitor will charge to a higher valuesubsequentpulse farrives the Bubsequentpositive pulsesoi the signal voltage d 'will haveno further conducting. The output voltage at terminal 43 (curve e) willtherefore remain constant until the start ofthe second cycle. when thesecond positive pulse 49 of the differentiated' control wave b willinitiate a repetition of the previous cycle oi operation. .Themtput'voltage of the upper envelope multivibrato"33" will therefore have theform of a square wave voltage e as shown in Fig. 2.

The output of the upper envelope multivibrator is differentiated by anupper envelope dierenof which will be as illustrated in curve j of Fig.2. This diilerentiated voltage f is applied to one input terminal 51 ofan upper envelope D. C. setterg, the circuit diagram of whichv is shownin Fig. 6. The D. C. setter is a device commonly employed in televisioncircuits for producing a D. C. output voltage whose amplitude isdetermined by the amplitude of a varying voltage at a particularinstant'oftime. The varying voltage is the symmetrical sawtooth voltagec which is applied to input terminal '55, while the output voltage f ofthe upper envelope dinerentiator is applied to input terminal 51 The D.C. setter comprises fo'urdiode rectiilers I9, il, I3 and The inputvoltage is supplied -between ground and the cathode of rectifier Sl andthe anode of rectiiier 6|. Output is taken across a capacitor l which isconnected between ground and output terminal 89, which terminal is alsoconnected to the cathode of rectier 03 and the anode of rectiiier Si.The anodes of rectifier 5l and areconnected together. as are thecathodes of rectiers tl and .65. The'input f from differentiator 5l isapplied to the primary of transformer 1|.the secondary of which isconnected between the common anodes of rectiilers Il and Il and thecommon cathodes of rectiilers Il and l5 through a parallel connectedresistor and capacitor. j

The positive pulses of the dierentiated voltage f cause a current toflow from the secondary of transformer 1| through the series resistorand through two parallel paths, the first comprising rectiilers 5.9 and8l and the second comprising rectiflers r:63 and 05. As a result anegative bias potential is developed across the capacitor, which issuiiicient to prevent the rectiers from conducting current in tential ofgreater amplitude applied to input terminal 51. Thus, after a suflicientto build up such a bias voltage, there is an extremelyhigh impedancebetween the input terminal 5I and the youtput/terminal 69, since theanodes of .,the four rectiilers are at a negative potential with respectto their cathodes.k However, at the instant a suillciently largepositive pulse f is applied to input terminal 51, the negative biasvoltage is overcome andthe Arectiilers then present anextremely lowimpedance to the ilow of current from the input terminal $5 to theoutput terminal 09. During this instant, therefore, capacitor 61 willbecome charged to a value depending upon the amplitude of the sawtoothvoltage c which is applied to input terminal l5. Il will retain thisvoltage until a subsequent time when asuccessive positive pulse inputterminal 51. If, at the time such a subsequent pulse is applied, thesawtooth voltage has increased,

0|, ,the resistor, transformer 1I at the time the amplitude of thethroughrectiner and rectifier 03. If, however,

the absence of a positive po-V ei'fect, since tube 3l is already shortperiod of operation c sawtoothvoltagecislessthanitlvl'evlloullirliadAvoltage f is applied. This voltage is shown in curve a ci Fig. 2.Inillustratlng prior to the cycle of operation illustrated the outputvoltage a was reduced to the relatively low value shown. Since in thecase illustratedthe top of che 1etr'a.'.' of Fig. 4, ls the me elstancefrom the starting point in both cycles illusv in the amplitude of thetrated, therel is no change D. C. control voltage a at the beginning ofthe second cycle of operation. Also, 'since the mirror is| vibratingsynchronously with the sawtooth voltage c, it will be appreciated thatthe amplitude of the D. C. control voltage a is a measure of thedistance from the starting point 2l to the edge of the top of theletter. It should further be notedthat the return path of the spot frompoint 23 to point 2l has no eilect on voltage g. This is because thesignal pulses ocof the, letter or curve curring in the second half ofthe first cycle of operation have no effect, since the multivibratorvoltage output e remains constant during this interval. Consequently. asthe probe is moved slowly along the line'of print,.the amplitude of theD. C. voltage g is proportional to the contour of the top of the letter.l 1

If desired, this D. C. voltage y may be used in any convenient manner torepresent fthe contour being scanned. In the particular case, where thedevice is -to be used as a reading aid for the blind. it is preferred toconvert the variable amplitude D. C. voltageinto an audible tone whosefrequency isproportional to the amplitude of the D. C. voltage andtherefore proportional to the contour of the letter. This may beaccomplished by applying the voltage a to an upper envelope low passvillter 13. which is simply a resistance-capacitance network, to smoothout the sudden variations in voltage which may be produced. The lteredvoltagey is then applied to the input of an upper envelope frequencymodulated oscillator 15. This device may be of any conventional type,suchasone employing'a reactance tube or a saturable reactor to modulatethey frequency of omillation.v The frequency range of oscillationpreferably does not exceed'one octave and may be of the order of from1000 to 2000 cycles. However, it may have any convenient range otfrequencies which are readily dlstinguishable. The output of thefrequency modulated oscillator is then passed through a band pass illter11 to remove any harmonic or distortion component frequencies and thenapplied to a mixer 19. if necessary, where it is combined with othervoltages which may be developed in a manner to be described subsequentlyand then to output device 8|, such as a loudspeakercurvemithasbeenassuniedthat` the amplitude of l* held in a stationaryposition over the letter.y Furthermore, the `cyclic interruption of thetone created harmonic frequencies which made it diiiicult to identifythe exact range of the tone. These facts made it veryv diilicult toidentify the character. In the present system, if the probe or scanneris held stationary over the letter, the output tone is a continuous noteof a given frequency, containing no harmonics andtherefore readilyidentified. As the probe is moved slowly along the printed line, in thepresent case, the audible tone varies slowly in frequency in accordancewith the distance from the starting line to the top of the letter at thepoint being scanned, and

thus the slowly varying frequency of the audible tone is a directindication of the outline or contour of the top of the letter as theprobe is moved along the line of characters.

While the system described so far has many uses, particularly inconnection with the determination of the contour of a function-curve incomputers ,as discussed above, in its particular application t a readingaidY for the blind,the

by providing a simultaneous indication of the contour of thelower edgeof the letters.

l I I; v I' 'I L passed through a suitable band pass lter. Il to themixer 19 where its relative amplitude can be controlled as desired, andthen to theoutput device 0|.

Referring now to Fig. 3, a number of Ithe lower?. case letters of thealphabet have been illustrated,`

together with a chart showing the frequencyof the output tonesrepresentative of the upper and lower contours of the letters. Byobservation it may be seen that most of the letters are clearlyidentifiable and thus the user will be able to .recognize the differentcharacters either directly in terms of the vcharacteristic variations infrequency of the two tones, or by forming a mental image of the contourof the characters in accordbe described in which a third audible toneispro;-

In accordance with the present invention, the

lower contour is indicated by means of additional componentssubstantially identical to those deof a polarity reverser 03. This maybe merely an amplifier tube, or the same function may be provided byreversing the voutput connections of the diierentiator I1, if the outputsystem is balanced with respect to ground. The wave of reversed polarityis illustrated in curve br in Fig. 2. This voltage is applied toterminal 4| of the lower .envelope multivibrator 85, the circuit diagramof which is the same as that illustrated in Fig. 5. The signal voltage dis applied to the other input terminal 39 as in the preceding case. Thusthe output voltage h of Fig. 2 is similar to voltage e except that thereversal occurs during the second half cycle of operation. This voltageis then differentiated by a device 91, which' is similar to device 5|,and applied to a lower envelope D. C. setter 09 which is identical tothe upper envelope D. C. setter 53, the circuit diagram being asillustrated in Fig. 6. Curve i of Fig. 2 therefore represents the D. C.voltage output of the device 09, and it will be observed that thisvoltage varies in amplitude as a function of the distance between thelower limit of the movement of the spot, the points 23, 29, etc., andthev lower edge of the letter at the point in question. This voltage isthen filtered by a lower envelope low pass filter 9| and applied tolowerenv'elope frequency-modulatedy oscillator 93, the frequency rangeof which is preferably outside the range utilized for the upper envelopeoscillator. For example, the lower envelope oscillator may be modulatedfrom 100 to 200 cycles per second. This voltage may be series limitingresistor |0I to the grid of Itube |03. '.The cathode of this tube isconnected tothe of the input voltage becomes.

duced which is keyed on only when part of a letter which occupies anarea near the horizontal identification of the characters can beimprovecL 30 center line is encountered by the spot of light. I.

In order to accomplish the center line identi-1 fication the saw toothvoltage c is applied to a limiter or clipper 91 which may be of the typeillustrated in Fig. 7. The sawtooth voltage is applied to an inputterminal 99 and through a cathode of a second 'tu-be |05 and bothcathodes are grounded through v`a series resistor |01 and an adjustablebias source |09. The plate of tube |03 is connected to a suitable sourceof positive potential, while the plate of tube |05 is connected to thesame source through a load resistor I.

The grid of tube |05 is connected to the unf a predetermined positivevalue, the plate currentI flowing through resistor |01 willproduce apositive voltage suflicient to bias oil.' tube |05. The output voltagethen is` approximately equal t0 the applied plate voltage, and remainsfixed at this value without regard .to the amplitude of the inputvoltage. When the amplitude of the sawtooth voltage decreases below apredetermined value tube |03 will be cut off, the output tube |05 willdraw a fixed 'current and the output voltage will drop to a valuedetermined by the plate voltage and the resistance of the load resistorI The output voltage will remain at this value no matter how muchgreater the negative amplitude The purpose of the Iadjustable bias |09is to s et the operating point at approximately the mid-value of lthesavf tooth voltage as illustrated in curve 7c of Fig. 2. Ii the sawtoothvoltage is symmetrica1 with re-4 wave voltage occurs at a timecorresponding to applied to one input terminal lit of a keyer lll, whichmay be of the type illustrated in Fig. 8. The keyer is simply aconventional tube or the mixed type commonly used in superheterodynereceivers having at least two grid electrodes adapted to independentlycontrol the conductivity of the tube. Input terminal il! to which thevoltage l is applied is connected to one grid while the other grid isconnected to input terminal l2l. Signal voltage d from the amplifier I9is applied .to input terminai |2I. Output is taken across the cathodeload resistor |23. i

In the operation of this device an output voltage is produced onlyduring the period when both grids are driven in a positive direction.Projecting the positive pulses of curve d on to curve l, it will beobserved that curve m represents the output voltage during the rst twocycles illustrated in Fig. 4. That is, when the spot crosses the centerportion of the character E, for example, a signal indicating a blackarea in this region is produced. This signal coincides in time with thecenter keying pulse of voltage l and therefore an output voltage isproduced. During the second cycle of operation illustrated inFlg. 4,however, the spot misses lthe central portion 21 of the character E andit will be observed from the right hand portion of-curve m that nosignal is produced indicative of the existence` of a portion of acharacter along theA center line. The output of the keyer is applied toa peak detector |25, which may be of the type illustrated in Fig. 9. Thevoltage is applied to input terminal |21 which is coupled by capacitor|29 to the anode of a rectiiler ISI, the cathode of which is grounded.The anode is also connected to output terminal |33 and to a condenser|35 through a resistor |31.

In the operation of the peak detector the capacitor is charged to avoltage corresponding to the peak amplitude of the applied pulse. Thusif the probe is held stationary over the letter E so that the spot scanscontinuously the central portion 21, a continuous D. C. current lsproduced which is used to control the output of -a keyed audio frequencyoscillator |39. The frequency of this oscillator is constant, and may beany clearly distinguishable frequency, preferably not within the rangeof frequencies employed in the other oscillators and may be, forexample, 500 cycles. Thus, as long las the spot continues to strike acentral blank area a continuous 500 cycle Atone is produced. I-I-owever`when the probe moves to a position such that the spot does not encountera black area in the central region of the character, the condenser |35discharges. the output voltage of the peak detector drops to zero andthe output of the keyed oscillator |39 is likewise reduced to zero. Thisaudio frequency tone is also applied to mixer 19 and thus -to the outputdevicel.

In Figs. 3 land 3a, the existence of a central black portion of eachcharacter is indicated by the discontinuous line passing through thecen-1 ter of the contour curves, and it may be observed that thosecharacters which were previously identical as to their contours may nowreadilybe identified.

WhatIciaim is:

1. A device for producing an electrical signal corresponding to thecontour of an indicium com- The voltage l is then in this period. Thepulse is therefore the center of the distance traversed by the spot. l

. l0 prising means for cyclically moving a point source of energyprogressively in a direction normal to said transver-se direction,ducing a signal voltage corresponding to the energy reflecting propertyof said indicium from point to point along the path of saidpoint'source, and means initiated concurrently with .the start of thetransverse movement of said point source and terminated' .by the firstsubsequent signal voltage for producing a control voltage whoseamplitude is proportional to .the distance between the starting pointand the point atwhich said first subsequent signal voltage occurs(-vtion and the point at which said 2. A device of the character describedin claim* 1 which includes means for producing an output voltage whoseAfrequency varies as a function of the amplitude of said control voltage.

3. A device for producing an electrical signal corresponding to thecontour of an indicium comprising means for cyclically moving a pointsource of energy transversely across said indicium and progressively ina direction normal to said transverse direction, energy responsive meansfor producing a signal voltage corresponding to the energy reflectingproperty of said indicium from point to point along the path of saidpoint source. means initiated concurrently with the start of thetransverse movement of said point source in' one direction for producinga iirst control voltage whose tance between the starting point in saidone direcfirst subsequent signal voltage occurs, and means initiatedconcurrently with the start of the transverse movement of said pointsource in the other direction for producing a second control voltagewhose .amplitude is proportional to the distance between the startingpoint in said other direction and the point at which the next subsequentsignal voltage Occurs.

4. A device of the character described in claim 3 which includes; inaddition, means for producing twooutput voltages whose frequencies varyin accordance with the amplitudes of ,said control voltages,respectively.

5. A device for producing an electrical signal corresponding to thecontour of an indicium comprising means for cyclically moving a spot oflight transversely across s aid indicium and progressively in adirection normal to saidtransverse direction, light responsive means forproducing a signal voltage corresponding in amplitude to the amount oflight reflected from said indicium from point to point along the path ofsaid spot, a device having two conditions of stability for producing asquare wave voltage, means for conditioning said device to one of saidconditions concurrently with'the start of the trans- 6. A reading devicefor producing an audible tone indicative of the contour of the lettersof a line of print comprising means for cyclicaily scanning said linewith a spot of light vibrating transversely across the line and adaptedto be moved along the length lof said line, and means .transverselyacross said indicium and energy responsive means for proamplitude isproportional to the disresponsive to changes in the-intensity of thelight reflected from said line for producing an audible tone whosefrequency corresponds to the distance traversed by said spot from itsstarting point in a given direction to the first point of intersectionwith a letter. Y

7. A reading device for producing audible tones indicative of thecontour of the letters of a line of print which comprises means forcyclically scanning said line with a spot of light vibratingtransversely across the line and adapted to be moved along the length ofsaid line, light reofits conditions .concurrently with the` start ofvthe transverse movement of said spot in the opposite direction and forconditioning said other device to vits other condition in response tothe rst subsequent change in amplitude of said signal voltage, meanscontrolled by said square wave voltages'for producing two controlvoltages whose amplitudes are proportional, respectively, to thesponsive means for producing-a signal voltage,l

corresponding in amplitude to the amount of whose amplitude isproportional to the distance between the starting point of said spot andthe point at which said iirst subsequent change in amplitude of saidsignal voltage occurs.

8. A device of the character described in claim 7 which includes meansfor producing an audible tone and means for varying the frequency ofsaid tone in accordance with the amplitude of said control voltage.

9. A readingdevice for producing audible tones indicative of the upperand lower contours of the letters of a line of print which comprisesmeans for cyclically scanning said line with a spot of light vibratingtransversely across the line and adapted to be moved along the length ofsaid line, light responsive means for producing a signal voltagecorresponding in amplitude to the point along the path of said spot, apair of devices each having two conditions of stability for producingtwo square wave. voltages, means for conditioning one of said devices toone of said conditions concurrently with the start of the transversemovement of said spot in one direction, and for conditioning said deviceto the other condition in \response `to the rst subsequent change inamplitude of said signal voltage, means for conditioning the other ofsaid devices to one amount of light reflected from said line atanydistances between said starting points and the point at which saidiirst subsequent changes occur, and means for producing twoaudible toneswhose frequencies vary in accordance with the amplitudes of said controlvoltages, respectively.

10. A device of the character described in claim 9 which includes, inaddition, means for producing `a third audible tone of constantfrequency, and meansfor controlling the amplitude of said third tone inaccordance with the intensity of light reected from said line at a pointmidway between the limits of the lateral movement of said spot.

11. A reading device for producing an audible tone which varies infrequency as the device is moved along the line in accordance with thecontour of' the letters forming said line which in:v

cludes' means for cyclically scanning ,said line with a spot of iightvibrating transversely across the line, light'responsive means receptiveto light reflected from the line of print for 4producing a signalvoltage whose amplitude varies in accordance with the black and whitearea on which the spot falls at any instant, a multivibrator having twostable conditions of operation forv producing a square wave voltage,means for producing a voltage pulse concurrent with the start `oi? thetransverse movement of said spot in a given di# rection, means forapplyingsaid pulse and said signal voltage to said multivibrator tocontrol its condition of operation, means for deriving from said squarewave voltagea` pulse which coincides with the nrst change in amplitudeof said signal voltage in each cycle of vibration oi said spot, meansunder the control oi said pulse for producing a D. C. voltage whoseamplitude is proportional to the distance traversed by said spot up tothe point where said first change in amplitude occurs, a source ofaudible frequency oscillations and means for controlling the frequencyof said oscillations in accordance with the amplitude of said D. C.voltage.

i ALDA V. BEDFORD.

No references cited.

